According to the statistics of China Disabled Persons' Federation in 2012, China has more than 20,000,000 amputees. Limb loss has brought them lots of inconveniences to their daily living and deteriorate their life quality. Lower-limb prostheses help the transtibial amputees restore their ability to stand and walk, and can significantly improve their life quality. Currently the commercial transtibial prostheses can be classified into two categories according to the ankle angle adjustment ability: the fixed-ankle prostheses and the adjustable-ankle prostheses. Fixed-ankle prostheses can help the wearer to support their body, but cannot mimic the dynamic characteristics of the normal ankle. Adjustable-ankle prostheses can adjust the ankle angle within a predefined range as the normal ankle, but their ankle impedance is usually determined by the mechanical structure and prosthesis material, and cannot be adjusted according to different walking speeds and terrains. Consequently, amputees may suffer from asymmetric gait patterns and decreased walking stabilities when walking on ramps and stairs with these kinds of prostheses.
To deal with speed and terrain variations, the ankle should be able to modulate the joint impedance. Patent CN101785716B has presented a hydraulic bionic ankle prosthesis, which controls the opening size of the oil fluid tube with a throttle valve, and consequently controls the flow resistance of the fluid and the joint rotation impedance. Patent CN101856283B has presented a pneumatic ankle prosthesis, which controls the opening size of an air cylinder with a stepping motor to control the joint impedance. Both the hydraulic and pneumatic prostheses above can control the ankle impedance, but it requires adding damping control components in addition to the main mechanical structure, which makes the prosthesis design more complicated. Moreover, there are risks of fluid or air leakage for these kinds of designs during long time operation.